Impelment autoprepare for extended protocol

Author: knizhnik

doc/src/sgml/autoprepare.sgml

@@ -0,0 +1,62 @@
+<!-- doc/src/sgml/autoprepare.sgml -->
+
+ <chapter id="autoprepare">
+  <title>Autoprepared statements</title>
+
+  <indexterm zone="autoprepare">
+   <primary>autoprepared statements</primary>
+  </indexterm>
+
+  <para>
+    <productname>PostgreSQL</productname> makes it possible <firstterm>prepare</firstterm>
+    frequently used statements to eliminate cost of their compilation
+    and optimization on each execution of the query. On simple queries
+    (like ones in <filename>pgbench -S</filename>) using prepared statements
+    increase performance more than two times.
+  </para>
+
+  <para>
+    Unfortunately not all database applications are using prepared statements
+    and, moreover, it is not always possible. For example, in case of using
+    <productname>pgbouncer</productname> or any other session pooler,
+    there is no session state (transactions of one client may be executed at different
+    backends) and so prepared statements can not be used.
+  </para>
+
+  <para>
+    Autoprepare mode allows to overcome this limitation.
+    In this mode Postgres stores plans for all queries pass using extended protocol.
+    Speed of execution of autoprepared statements is almost the same as of explicitly prepared statements.
+  </para>
+
+  <para>
+    By default autoprepare mode is switched off. To enable it, assign non-zero
+    value to GUC variable <varname>autoprepare_limit</varname> or <varname>autoprepare_memory_limit</varname>.
+    This variables specify limit for number of autoprepared statement or memory used by them.
+    Autoprepare is enabled if one of thme is non zero. Value -1 means unlimited.
+    Please notice that event autoprepare is anabled, Postgres makes a decision about using
+    generalized plan vs. customized execution plans based on the results
+    of comparison of average time of five customized plans with
+    time of generalized plan.
+  </para>
+
+  <para>
+    Too large number of autoprepared statements can cause memory overflow
+    (especially if there are many active clients, because prepared statements cache
+    is local to the backend). So using unlimited autoprepare hash is debgerous and not recommended.
+    Postgres is using LRU policy to keep in memory most frequently used queries.
+  </para>
+
+  <para>
+    Autoprepare hash is local to the backend. It is implicitely reseted on any change of database schema or
+    session variables.
+  </para>
+
+  <para>
+    It is possible to inspect autoprepared queries in the backend using
+    <literal>pg_autoprepared_statements</literal> view. It shows original text of the
+    query, types of the extracted parameters (replacing literals) and
+    query execution counter.
+  </para>
+
+ </chapter>

doc/src/sgml/config.sgml

@@ -5326,6 +5326,39 @@ SELECT * FROM parent WHERE key = 2400;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-autoprepare-limit" xreflabel="autoprepare_limit">
+       <term><varname>autoprepare_limit</varname> (<type>integer/type>)
+           <indexterm>
+             <primary><varname>autoprepare_limit</varname> configuration parameter</primary>
+           </indexterm>
+       </term>
+       <listitem>
+         <para>
+           Maximal number of autoprepared queries.
+           Zero value disables autoprepare, -1 means unlimited number of autoprepared queries.
+           Too large number of prepared queries can cause backend memory overflow and slowdown execution speed
+           (because of increased lookup time). Default value is <literal>0</literal>.
+         </para>
+       </listitem>
+     </varlistentry>
+
+     <varlistentry id="guc-autoprepare-memory-limit" xreflabel="autoprepare_memory_limit">
+       <term><varname>autoprepare_memory_limit</varname> (<type>integer/type>)
+           <indexterm>
+             <primary><varname>autoprepare_memory_limit</varname> configuration parameter</primary>
+           </indexterm>
+       </term>
+       <listitem>
+         <para>
+           Maximal size of memory used by autoprepared queries.
+           Zero value disables autoprepare, -1 means means that there is no memory limit.
+           Default value is <literal>0</literal>. Calculating memory used by prepared queries adds some extra overhead,
+           so positive value of this parameter may cause some slowdown.
+           <varname>autoprepare_limit</varname> is much faster way to limit number of autoprepared statements.
+         </para>
+       </listitem>
+     </varlistentry>
+
      </variablelist>
     </sect2>
    </sect1>

doc/src/sgml/filelist.sgml

@@ -22,6 +22,7 @@
 <!ENTITY json       SYSTEM "json.sgml">
 <!ENTITY mvcc       SYSTEM "mvcc.sgml">
 <!ENTITY parallel   SYSTEM "parallel.sgml">
+<!ENTITY autoprepare SYSTEM "autoprepare.sgml">
 <!ENTITY perform    SYSTEM "perform.sgml">
 <!ENTITY queries    SYSTEM "queries.sgml">
 <!ENTITY rangetypes SYSTEM "rangetypes.sgml">

doc/src/sgml/postgres.sgml

@@ -109,6 +109,7 @@
   &mvcc;
   &perform;
   ∥
+  &autoprepare;
 
  </part>
 

src/backend/catalog/system_views.sql

@@ -332,6 +332,9 @@ CREATE VIEW pg_prepared_xacts AS
 CREATE VIEW pg_prepared_statements AS
     SELECT * FROM pg_prepared_statement() AS P;
 
+CREATE VIEW pg_autoprepared_statements AS
+    SELECT * FROM pg_autoprepared_statement() AS P;
+
 CREATE VIEW pg_seclabels AS
 SELECT
     l.objoid, l.classoid, l.objsubid,

src/backend/commands/prepare.c

@@ -48,7 +48,6 @@ static HTAB *prepared_queries = NULL;
 static void InitQueryHashTable(void);
 static ParamListInfo EvaluateParams(PreparedStatement *pstmt, List *params,
 									const char *queryString, EState *estate);
-static Datum build_regtype_array(Oid *param_types, int num_params);
 
 /*
  * Implements the 'PREPARE' utility statement.
@@ -787,7 +786,7 @@ pg_prepared_statement(PG_FUNCTION_ARGS)
  * pointing to a one-dimensional Postgres array of regtypes. An empty
  * array is returned as a zero-element array, not NULL.
  */
-static Datum
+Datum
 build_regtype_array(Oid *param_types, int num_params)
 {
 	Datum	   *tmp_ary;